Magnetic skyrmions are topologically protected spin textures in which the local moments on a two dimensional lattice point in all directions with a topologically nontrivial mapping to the unit sphere. Physically, each skyrmion is a circular spin texture in which the spins on the periphery are polarized vertically, the central spin is polarized in the opposite direction, and, in between, the spins smoothly transition between the two opposite polarizations. A swirling transition, which is effectively a circle of double Bloch-type domain wall, gives a Bloch-type skyrmion. This type of skyrmion was first discovered in the temperature-magnetic field (T-H) phase diagram of B20 magnets. In these materials, the atomic structure of the lattice breaks the inversion symmetry, inducing an asymmetric Dzyaloshinsky-Moriya (DM) exchange interaction. The competition between the DM exchange and the symmetric Heisenberg exchange stabilizes the skyrmion phase in these materials. A Néel-type skyrmion, on the other hand, is a wrapped double Néel-wall. Such a skyrmion is stabilized by an interfacial DM interaction, which is originated from the broken interfacial inversion symmetry. This type of DM interaction is usually observed at the interface between a magnetic thin film and a layer of heavy metal with strong spin-orbit coupling (SOC). For both types of skyrmions, the radius, ranging from about 3 nm to 100 nm, is determined by the ratio of the strengths of the DM interaction and the Heisenberg interaction. Skyrmion lattices and isolated skyrmions in both bulk and thin films have been observed by neutron scattering, Lorentz transmission electron microscopy, and spin-resolved scanning tunneling microscopy (STM). Current can drive skyrmion spin textures with a current density 4-5 orders of magnitude lower than that required to move conventional magnetic domain walls. This suggests promising spintronic applications exploiting the topological spin texture as the state variable. A two-dimensional skyrmion lattice may be formed under a uniform magnetic field, however, the switching of isolated, individual skyrmions is far more challenging. The single skyrmion switching was first experimentally demonstrated by injecting spin-polarized current from an STM tip into ultra-thin Pd/Fe/Ir(111) films of about several atomic layers (between about 1-2 nm) at 4.2 K schemes of single skyrmion switchings, such as using a sharp notch, a circulating current, thermal excitations and spin-orbit torques (SOTs) have been proposed. Spintronic applications call for on-wafer solutions to precisely control the position and the time of skyrmion switchings with good reliability. This is rather difficult because each switching event corresponds to a topological transition, which has to break the protection given by the topological order. This process has to overcome the topological protection barrier, which is both energetically unfavorable and difficult to manipulate.
Magnetic skyrmions are topologically protected, particle-like spin textures. They can generally range in size from 1 nm to approximately 100 nm depending on material parameters. Further, skyrmions can generally be created and annihilated by spin currents and magnetic fields. Skyrmions can also generally be moved by an electrical current.
In many materials, skyrmions can be the middle phase of a progression of three phases with increasing magnetic fields: helical, skyrmions, and ferromagnetic. Due to the small size of skyrmions, their stability, the demonstration of their individual creation and annihilation, and their facile movement by low currents, skyrmions can potentially be used for application such as information storage (memory).